1. Field of the Invention
The present invention relates to an image forming optical system, particularly an image forming optical system which has a high zooming ratio and a superior image forming performance while being of a slim type which is suitable for an electronic image pickup optical system, and an electronic image pickup apparatus having such image forming optical system.
2. Description of the Related Art
A digital camera has reached a level of being used practically, regarding making large the number of pixels (high image quality), and small-sizing and slimming. Also from a function point of view and a market point of view, a digital camera has substituted a silver salt 35 mm film camera. Therefore, a high zooming ratio and further increase in the number of pixels with the same small-size and slimness, have been sought strongly as a next trend of evolution.
A so-called positive-lead type with four or more lens groups has hitherto been used generally, as it is more effective for high zooming. However, most of the optical systems of the positive-lead type have a telephoto ratio exceeding one, and cannot be said to be of sufficiently small size.
For making an optical system of positive-lead type small, shortening of an overall length at a telephoto end is necessary. However, when the overall length of an optical system is shortened at the telephoto end, a negative curvature of field is developed. Therefore, the image forming performance is not satisfactory.
For achieving both of the small-sizing and the correction of the curvature of field, introducing a convex lens having a lower refractive index in a second lens group of the optical system of the positive-lead type is a conceivable means.
For instance, in a first embodiment of Japanese Patent Application Laid-open Publication No. 2008-203453, an arrangement is such that a refractive index of a convex lens is the smallest among all lenses in a second lens group.
Moreover, for example, in Japanese Patent Application Laid-open Publication No. 2009-9121, an arrangement is such that a positive lens having the smallest refractive index is arranged as a lens in a second lens group.